Method for operating an internal combustion engine having a steam power plant

ABSTRACT

A method for operating an internal combustion engine ( 10 ) having a steam power plant ( 12 ), in which the exhaust gas of the internal combustion engine ( 10 ) at least indirectly heats the steam and this steam drives a turbine ( 26 ), the steam power plant ( 12 ) being sometimes, in particular at the beginning of operation, initially operated in a protection operating mode, in which the protection of the turbine ( 26 ) from damage by water droplets has priority.

BACKGROUND OF THE INVENTION

The invention relates to a method for operating an internal combustionengine having a steam power plant, in which the exhaust gas of theinternal combustion engine at least indirectly heats the steam and thissteam drives a turbine, and to a control and/or regulating unit, acomputer program, and a storage medium.

Steam power plants, which utilize the waste heat in the exhaust gas ofan internal combustion engine, are known. DE 10 2006 057 247 A1describes a steam power plant, a heat exchanger of a circuit of anoperating medium being housed in an exhaust system. The heat exchangeris connected downstream from a conveyor assembly in the circuit of theoperating medium. The circuit of the operating medium contains a turbinepart, via which at least one compressor part situated in the intakesystem of the internal combustion engine is driven.

SUMMARY OF THE INVENTION

The invention has the advantage that in a steam power plant whichutilizes the waste heat in the exhaust gas of an internal combustionengine, a steam turbine is protected from damage by liquid droplets, inparticular water droplets, in the steam jet. In addition, rapidoperational readiness of the steam power plant is produced in order toprovide steam circuit power as early as possible and thus contribute toreducing the fuel consumption.

For example, a steam power plant in the exhaust system of the internalcombustion engine is constructed as follows: The exhaust gas flowsthrough a primary system of the heat exchanger. In the secondary system,the condensed water conveyed by a feed pump is heated in such a way thatsteam arises. The steam generated in the heat exchanger is conductedinto a turbine, where it expands in a nozzle and drives a rotor of theturbine. An outlet of the turbine conveys the expanded steam into acondenser. The expanded steam is cooled in the condenser, so that itcondenses to form water. This water is sucked in by the feed pump andsupplied again to the heat exchanger.

The invention proceeds from the consideration that steam turbines may bedamaged if water droplets condensed from the steam hit the blading ofthe rotor of the steam turbine at high velocity. This danger existsespecially during a startup procedure, when parts of the steam circuitand the steam turbine itself have not yet reached their respectiveoperating temperature. It is therefore proposed that the steam powerplant be operated during a startup procedure in a protection operatingmode, in which the protection of the turbine from damage by waterdroplets has priority. In such a protection operating mode, for example,it can be ensured as much as possible that the vapor generated in theheat exchanger cannot condense on cold pipe walls downstream. The dangeris therefore at least reduced that individual droplets will be entrainedby the steam flow and strongly accelerated, whereby damage could occurupon their impact on the rotor.

A first bundle of measures has the purpose that a relative velocitybetween an entering steam jet and a rotor of the turbine is reduced inthe protection operating mode in relation to a normal operating mode.The relative velocity characterizes the impact velocity of the waterdroplets on the rotor of the turbine. The lower the relative velocity,the lower the probability of damage to the rotor. In this way, thedurability of the turbine and therefore of the steam power plant as awhole is advantageously increased.

For this purpose, the method according to the invention provides thatthe relative velocity between the incoming steam jet and the rotor ofthe turbine is reduced, in that at least sometimes a cross section of anozzle for controlling the incoming steam jet is enlarged and/or theturbine is operated without a load and/or the rotation of the turbine issupported by a motor. If the cross section of the nozzle is enlarged, alower pressure ratio results via the nozzle, which in turn results in alower acceleration and a lower exit velocity of the steam jet.Additionally or alternatively, the relative velocity between the steamjet and the rotor of the turbine can be reduced further if the turbineis operated without a load, i.e., can rotate freely. Additionally oralternatively, the relative velocity can in turn be reduced further ifthe turbine is supported by a motor. For example, the rotor can becoupled to an electric motor which synchronizes the rotor of the turbineat least approximately to the steam jet velocity. Damage to the rotorcan thus be avoided in three ways. The electric motor can advantageouslyoperate as a generator to generate electrical energy in the followingnormal operating mode of the steam power plant.

A second bundle of measures has the purpose that a temperature of thesteam generated in a steam generator is increased up to a limitingtemperature above a normal operating temperature in the protectionoperating mode in relation to the normal operating mode. While theabove-described first measures reduce the relative velocity of the waterdroplets condensed from the steam in relation to the rotor, the secondmeasures have the effect that water droplets in the steam jet are formedeither not at all or only in a reduced quantity, i.e., a backcondensation of the steam jet essentially does not occur. Both bundlesof measures may supplement one another and advantageously add theireffects together.

For this purpose, the method according to the invention provides thatthe temperature of the steam generated in the heat exchanger (“steamgenerator”) is increased up to a limiting value above the normaloperating temperature, in that an exhaust-side bypass of a heatexchanger is closed or is reduced in its flow rate, if an operatingtemperature of the heat exchanger has not exceeded a maximum permissiblevalue, and/or a delivery quantity of a feed pump is adapted, and/or thecooling of a condenser is stopped or reduced, if an operating pressureof the steam power plant has not exceeded a maximum permissible value.It is thus possible to “superheat” the steam in relation to the normaloperating mode, so that the probability of the occurrence of the harmfulwater droplets is less. The limiting value for the temperature of thesteam is preferably selected in such a way that no part of the steampower plant is excessively strained or the safety is endangered. If theexhaust gas heat exchanger has an exhaust-side bypass, the bypass isinitially closed during the protection operating mode, so that theentire exhaust gas stream can flow through the heat exchanger. Atemperature of the heat exchanger is simultaneously monitored. As soonas an upper permissible operating temperature of the heat exchanger isreached or even exceeded, the bypass is opened, with the result that theexhaust gas quantity flowing through the heat exchanger is reduced. Aninner heat exchanger temperature is preferably used as a controlvariable for adjusting the bypass, which can also be ascertained, forexample, by a thermal heat exchanger model on the basis of inputvariables, such as for example an exhaust gas temperature, an exhaustgas mass flow, a fluid temperature, and a fluid mass flow.

Alternatively or additionally, a delivery quantity of the feed pump canbe regulated or adjusted. The feed pump conveys water into a fluid inletof the heat exchanger, so that the steam exit temperature from the heatexchanger can be adjusted using the delivery quantity. For example, inthe normal operating mode of the steam power plant, a fluid mass flowsupplied to the heat exchanger is between 8 g/s and 60 g/s (grams persecond). During the protection operating mode it can be advantageous toreduce the fluid mass flow accordingly in relation to these values,since energy is required for heating the heat exchanger, which is notavailable for the vaporization and superheating. The fluid mass flow canthus be adjusted and/or regulated using a speed change of the feed pumpin such a way that the steam temperature is increased in relation to thenormal operating mode. The temperature of the steam generated in theheat exchanger is between 270° C. and 360° C. in the normal operatingmode, for example.

Alternatively or additionally, the cooling of the condenser can beadjusted. For example, the cooling of the condenser can be stopped,whereby less water condenses, and a higher steam temperature issubsequently reachable in the heat exchanger. The operating pressure ofthe steam power plant is monitored simultaneously. If the maximumpermissible operating pressure is reached or exceeded, the cooling ofthe condenser is turned on again and/or continuously increased. Forexample, depending on the operating point of the internal combustionengine, the required cooling power of the condenser can be 19 kW to 140kW. The cooling power of the condenser is preferably not switched, butrather regulated. The permissible operating pressure of the steam powerplant is the control variable. In contrast, if as much exhaust gas heatas possible is to be transferred into the cooling system of the internalcombustion engine, to which the condenser is connected, the condenser isalready cooled at the beginning. This can be advantageous for shorteningthe warm-up or reducing the emissions of the internal combustion engine.In this case, for example, the cooling power on the condenser isregulated in such a way that an operating pressure of, for example, 2bar results. It is also possible to execute the regulation of thecondenser cooling with respect to the protection operating mode, on theone hand, and shortening the warm-up and reducing the emissions, on theother hand, i.e., to combine both requirements.

Furthermore, the method provides that the protection operating mode isended when the steam power plant has reached a normal operatingtemperature. The transition from the protection operating mode to thenormal operating mode can thus occur as rapidly as possible, because nodamage of the turbine by water droplets is to be expected at the normaloperating temperature. The criterion for changing over can either beobtained by various temperature and/or pressure sensors at variouspoints of the steam circuit, or can be computed by a thermal model. Innormal operation, process control of the steam circuit which isoptimized in efficiency is achieved.

Furthermore, the method provides that at least an operating pressure ofthe steam power plant, a pressure ratio between an inlet and an outletof the turbine, a fluid mass flow of a feed pump, a temperature of aheat exchanger between exhaust gas and steam, a temperature of acondenser, and/or a temperature of the turbine is monitored. Therefore,on the one hand, a reliable differentiation can be made between theprotection operating mode and the normal operating mode, and, on theother hand, the protection operating mode can be adjusted or regulatedoptimally. In this way, damage to the steam power plant can also beparticularly reliably prevented.

In addition, the method provides that when the pressure ratio betweenthe inlet and the outlet of the turbine reaches or exceeds a thresholdvalue, the delivery quantity of a feed pump is reduced. For example, ifthe pressure ratio exceeds a threshold value, the delivery quantity ofthe feed pump is reduced enough that the pressure ratio is again belowthe threshold value. This can prevent the steam exit velocity out of thenozzle of the turbine from being so great that the rotor of the turbinecould be damaged by water droplets. The threshold value for the pressureratio is specified accordingly.

Furthermore, the method provides that a superheating temperature of thesteam—if water is used as the operating means—in normal operation isbetween 270° C. and 360° C. Using a superheating temperature of thesteam determined by these limits, particularly good efficiency of thesteam power plant can be achieved in the normal operating mode.Proceeding therefrom, it is possible to increase the steam temperaturegenerated in the heat exchanger at the beginning of operation—i.e.,during the protection operating mode—in order to reduce or prevent theoccurrence of the harmful water droplets. The upper limit of thesuperheating temperature of the steam is determined by the temperaturecompatibilities of the components of the steam power plant and by safetyrequirements.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained hereafter withreference to the drawing. In the drawing:

FIG. 1 shows a schematic of a steam power plant; and

FIG. 2 shows a flowchart of a sequence of the method in a protectionoperating mode.

DETAILED DESCRIPTION

The same reference numerals are used for functionally equivalentelements and variables in all figures, even in the case of differentembodiments.

FIG. 1 shows a steam power plant 12, which is supplied with energy bythe exhaust system of an internal combustion engine 10. The exhaust gasof the internal combustion engine 10 is supplied via an exhaust pipe 14and a bypass valve, implemented in the present case as a three-way valve16, to a heat exchanger 18, which has the task of a steam generator 17.A third connection of the three-way valve 16 conducts exhaust gas via abypass 19 past the heat exchanger 18. The exhaust gas flows through aprimary system of the heat exchanger 18 and is then exhausted togetherwith the component conducted through the bypass 19 through an outletpipe 20.

A secondary system of the heat exchanger comprises a fluid entry 21 anda steam exit 22. The steam exit 22 leads to an inlet 24 of a turbine 26.The turbine 26 is implemented as an impulse turbine and comprises anozzle 28 and a rotor 30. An outlet 32 of the turbine 26 leads to acondenser 34. An outlet 36 of the condenser leads to an inlet 38 of afeed pump 40. The feed pump 40 operates at the fluid entry 21 of theheat exchanger 18, so that the steam circuit is closed.

The turbine 26 operates at a load 42 which is controlled by an actuator44. A dashed line 46 indicates that the load 42 can support a drive ofthe internal combustion engine 10. For example, the load 42 can be acompressor part, which increases a pressure in the intake system of theinternal combustion engine 10, so that during an intake stroke of theinternal combustion engine 10, a greater quantity of air required forthe combustion can reach the cylinder. Furthermore, an electric motor 48is coupled to the turbine 26. A control and/or regulating unit 50, whichcomprises a storage medium 52 and a computer program 54, is located onthe lower left in the illustration of FIG. 1. Dashed arrows 56 indicatethat the control and/or regulating unit 50 is connected to variouscomponents of the steam power plant 12.

Inter alia, the following are measured in operation of the steam powerplant 12: A temperature of the heat exchanger 18 using a temperaturesensor 58, a pressure at the inlet 24 of the turbine 26 using a pressuresensor 60, and a pressure at the outlet 32 of the turbine 26 using apressure sensor 62. Furthermore, an actuator 64 for adjusting thecooling of the condenser 34 and an actuator 66 for adjusting a speed ofthe feed pump 40 are provided.

If it is established by the control and/or regulating unit 50 that thesteam power plant 12 or the internal combustion engine 10 is in astartup state, a changeover is performed into a protection operatingmode. The control and/or regulating unit 50 obtains the information forthis purpose, inter alia, from the data of the pressure sensors 60 and62, the temperature sensor 58, and from the models of the steam circuitstored in the control and/or regulating unit 50. Multiple arrows shown(without reference numerals) in the circuit of the steam power plant 12indicate the direction of the fluid stream, the steam stream, and theexhaust gas stream. In other words, under certain circumstances, thesteam power plant is operated in the protection operating mode.

A plurality of measures is performed simultaneously in the protectionoperating mode: A cross section of the nozzle 28 of the turbine 26 isenlarged, so that the velocity of the incoming steam jet is reduced. Arelative velocity of the incoming steam jet in relation to the rotationof the rotor 30 is thus reduced. The actuator 44 decouples the load 42from the turbine 26. The electric motor 48 is energized and drives theturbine 26 in such a way that the rotor 30 is essentially synchronizedwith the velocity of the incoming steam jet.

Furthermore, various measures are performed in order to keep thetemperature of the steam at the steam exit 22 of the heat exchanger 18as high as possible. Firstly, the three-way valve 16 is set in such away that the exhaust gas stream exiting from the internal combustionengine 10 is conducted completely through the primary system of the heatexchanger 18, as long as a temperature monitored by the temperaturesensor 58 does not exceed a threshold value. If this threshold value isreached or exceeded, the three-way valve at least partially opens theroute via the bypass 19, in order to guide less exhaust gas through theheat exchanger 18 and thus keep its temperature below the thresholdvalue.

Secondly, using the actuator 66, the delivery quantity of the feed pump40 is adjusted in such a way (i.e., kept comparatively low at thebeginning of operation), that a desired steam temperature results at thesteam exit 22 of the heat exchanger 18. The delivery quantity of thefeed pump 40 is adjusted in the present case by a speed of the feed pump40. In addition, the delivery quantity of the feed pump 40 is adjustedas a function of a pressure difference between the inlet 24 and theoutlet 32 of the turbine 26. The pressure difference is determined by adifference in the pressures ascertained in the pressure sensors 60 and62.

Thirdly, the steam circuit is rapidly heated up in that the cooling ofthe condenser 34 is stopped or reduced as long as a permissibleoperating pressure of the steam power plant 12 has not yet been reached.If this operating pressure is reached, the cooling of the condenser 34is turned on and/or amplified.

If it is recognized on the basis of the temperature sensor 58 and thetwo pressure sensors 60 and 62 and a model stored in the control and/orregulating unit 50 that the startup procedure of the steam power plant12 is completed, a switch is made into a normal operating mode. The load42 is coupled to the turbine 26 and can support the drive of theinternal combustion engine 10. The electric motor 48 is either turnedoff or switched over into the generator operation. The remainingactuators of the steam power plant 12 are operated according to thenormal operating mode.

FIG. 2 shows a flowchart of the sequence of the method using a computerprogram 54 in the control and/or regulating unit 50. Starting from astarting block 70, it is queried in a block 72 whether the requirementsfor a protection operating mode are fulfilled. If not, the sequencebranches into an end block 74, in which a normal operating mode of thesteam power plant 12 is produced and the illustrated flowchart is thusexited.

However, if so, various variables of the steam circuit are ascertainedin a following block 76. These include, inter alia, the temperature ofthe temperature sensor 58, the pressures of the pressure sensors 60 and62, and various model variables 78 of the steam power plant 12 stored inthe control and/or regulating unit 50. In a following block 80, variousmeasures are performed to reduce the relative velocity between theincoming steam jet in the turbine 26 and the rotor 30 of the turbine.For this purpose, an actuator 82 (not shown in FIG. 1) is actuated inorder to enlarge the cross section of the nozzle 28. Furthermore, anactuator 44 is actuated by the block 80 in order to decouple the load 42from the turbine 26. Furthermore, the block 80 switches the electricmotor 48 via the actuator 48 in such a way that it is operated as amotor and adapts the speed of the rotor 30 to the velocity of theincoming steam jet through the nozzle 28.

Various measures are performed in a following block 84 in order toincrease the temperature of the incoming steam jet in the turbine 26over a normal value. For this purpose, the three-way valve 16 isadjusted using an actuator 86 in such a way that an exhaust gas streamthrough the bypass 19 is reduced or blocked. Furthermore, the deliveryquantity of the feed pump 40 is adapted via the actuator 66. The coolingpower of the condenser 34 is reduced via the actuator 64 as long as apermissible operating pressure of the steam power plant 12 is notexceeded. Subsequently, the program branches back to the block 72, whereit is checked again whether the condition for the protection operatingmode is fulfilled. If so, the sequence branches to block 76 again inorder to adjust or regulate the various variables, as is required forthe protection operating mode.

The method is terminated as soon as it is established in block 72 thatthe startup state has been ended. The sequence can then switch into thenormal operating mode in block 74.

1. A method for operating an internal combustion engine (10) having a steam power plant (12), in which the exhaust gas of the internal combustion engine (10) at least indirectly heats the steam and this steam drives a turbine (26), characterized in that the steam power plant (12) is, under certain circumstances, initially operated in a protection operating mode, in which the protection of the turbine (26) from damage by liquid droplets has priority.
 2. A method according to claim 1, characterized in that a relative velocity between an incoming steam jet and a rotor (30) of the turbine (26) is reduced in the protection operating mode in relation to a normal operating mode.
 3. A method according to claim 2, characterized in that the relative velocity between the incoming steam jet and the rotor (30) of the turbine (26) is reduced in that at least sometimes a cross section of a nozzle (28) for controlling the incoming steam jet is enlarged and/or the turbine (26) is operated without a load (42) and/or the rotation of the turbine (26) is supported by a motor (48).
 4. A method according to claim 1, characterized in that a temperature of the steam generated in a steam generator (17) is increased up to a limiting value above a normal operating temperature in the protection operating mode in relation to the normal operating mode.
 5. A method according to claim 4, characterized in that the temperature of the steam is increased up to a limiting value above the normal operating temperature, in that an exhaust-side bypass (19) of a heat exchanger (18) is closed or reduced in its flow rate, if the operating temperature falls below a permissible operating temperature of the heat exchanger (18), and/or a delivery quantity of a feed pump (40) is adapted, and/or the cooling of a condenser (34) is stopped or reduced, if the operating pressure falls below a maximum permissible operating pressure of the steam power plant (12).
 6. A method according to claim 1, characterized in that the protection operating mode is ended when the steam power plant (12) has reached a normal operating temperature.
 7. A method according to claim 1, characterized in that at least an operating pressure of the steam power plant (12), a pressure ratio between an inlet (24) and an outlet (32) of the turbine (26), a fluid mass flow of a feed pump (40), a temperature of a heat exchanger (18) between exhaust gas and steam, a temperature of a condenser (34), and/or a temperature of the turbine (26) is monitored.
 8. A method according to claim 1, characterized in that when the pressure ratio between the inlet (24) and the outlet (32) of the turbine (26) reaches or exceeds a threshold value, the delivery quantity of a feed pump (40) is reduced.
 9. A method according to claim 1, characterized in that a superheating temperature of the steam in normal operation is between 270° C. and 360° C.
 10. A computer program (54) characterized in that it is programmed for application in a method according to claim
 1. 11. A storage medium (52) for a control and/or regulating unit (50), characterized in that a computer program (54) for application in a method according to claim 1 is stored thereon.
 12. A control and/or regulating unit (50), characterized in that it is programmed for application in a method according to claim
 1. 13. A method according to claim 1, wherein the steam power plant (12) is operated in a protection operating mode at the beginning of operation.
 14. A method for operating an internal combustion engine (10) having a steam power plant (12), in which the exhaust gas of the internal combustion engine (10) at least indirectly heats the steam and this steam drives a turbine (26), the method comprising: determining if the steam power plant or the internal combustion engine is in a startup state; and, if so, operating the steam power plant in a protection operating mode.
 15. A method according to claim 14, further comprising: determining whether the startup state is completed; and, if so, operating the steam power plant in a normal operating mode. 